You should be able to tell if that is the issue with your multi-meter. If the float is at the bottom of the tank, you should get a resistance reading like the tank is empty (somewhere around 22.5 ohms). If you get a short to ground or an open then you know it is a wiring issue.

 

yep - exactly my thoughts. and for what its worth, i'm pretty sure I have the plastic float. I didn't really take note, but I think I would have noticed a brass one.

didn't get a chance to check it out last night, but i'm thinking maybe the float is dead, though. after I changed the tank, it took me about a week to complete my tank vent mod, during which time, I left the tank empty - so the float was probably bouncing off the bottom of the tank all along. maybe it split the plastic.

...all just conjecture now until I ohm it out.

thanks for the input guys, though!

 

If i understand this correctly a broken ground wire (or hot wire between the selector valve and the variable resistor for that matter) would cause the gauge to read full, like really full. So if its not the sending unit or float causing the problem, then it must be a short in the hot wire between the selector valve and the variable resistor or maybe something wrong in the selector valve.

 

^^^^ ... or the dark blue / yellow wire (ckt 673) between the tank switcher and the front sending unit is pinched to ground. Or the float has sunk as suggested above.

Test - follow the wire from the front tank to where it connects to the main harness on the frame. Disconnect it there. That should translate to infinite resistance, and peg the gauge to F . If it stays at E, then there's a short to ground between that point and the selector valve. If it pegs to F, then put an ohmmeter probe on the terminal for the wire going to the sender (where you disconnected it), other probe to ground. If it reads 22-ish ohms, then the float has probably sunk. If it reads 145-ish ohms, then the float is probably good. If it reads zero or near zero, then that wire is shorted to ground, somewhere between there and the tank. If it reads open/infinite, then that wire is broken.

 

Thats what I meant w/o the technical descriptions.

 

you guys are both right - I had it backwards - and my lunchtime testing just confirmed your theory. if the whole circuit is essentially a voltage divider circuit (a voltmeter with high resistance, i.e. the gauge, and the sending unit with low resistance - in series with each other), then, yes a broken or open wire would be infinite resistance in the circuit, causing the gauge to be pinned full instead of empty.

and with the connector plug unplugged that guage needle sure went to full - except there is no pin on that side to pin it to. so, it just kept going! like an inch past the F before I turned the key off afraid the gauge would break itself!

then I got to probing and found that the resistance between the signal wires on the sender unit (I forget the colors) was 15 ohms - which, according to the diagram would read VERY empty, like it does.

now, the wires looked in good shape from there to the sender, so here's my thinking:
- either the float is sunk, which is probably most likely, or
- the tank is so full that the float is pinned to the top of the tank and shorting the arm that connects the rheostat to the float back to the tank.

so, the plan is to just keep watching the triptometer (which I set to zero when I noticed the tank was full but gauge read zero), run the truck another 100 miles, which should be somewhere between 1/2 to 3/4 full (18 gallons at ~18mpg) and see if the gauge starts reading right - i.e. un-shorts itself. otherwise, out comes the sender and in goes the one from the other truck.

at least my lock ring is new and should come out much easier than the one from the old tank!

anyone know if the sender can come out with the tank in place, or should I plan on running it to empty and dropping the tank again?

 

Never tried removing the sender with the tank in place, but from having done the tank from below recently, and remembering the length of the pickup, I wanna say NO, it won't clear. You'd also have a fun time trying to tap loose the lock ring with just that little bit of clearance.

Can you remove the bed?....

Might wanna have a new lock ring on hand; yeah a new-ish one should actually turn loose, but they're basically made to be a once-and-done kind of part. But it's kinda like a throttle body gasket on a gasser; _not_ having a spare on hand insures that you _will_ spoil it upon removal (Murphy's Law...).

 

very true. do you think if I call the local pep boys and ask if they have one in stock before I try to tap the old one loose that will be enough? :-) that probably just means that when I do bugger mine that the one that pep boys said they have will be the wrong size!

but I was up under there today at work and with my truck as an F350 crew cab, it was actually pretty comfortable to sit on my butt indian-style and have good access to the top of the fuel tank. and the angle the sender lit is mounted on makes me thing it was perhaps designed to come out in place - though i'm sure i'll find out that there's just not quite enough room!

as for taking the bed off - doesn't that require dropping the front tank anyway?

 

Whuh, a FLAPS parts listing/database with an error? Imagine that....

Yuh, I was able to get the same access even under our F250; you have a couple more inches (of people room, that is). And maybe with that angle, you just might be able to swing it. (Or, in the process of tweaking and trying different angles, the showerhead will fall off and into the tank.... )

Not to my knowledge. I've never pulled the bed for this kind of work, just because we have access to only a single width of a shared driveway, and therefore no place to put the bed once I have it off. But I've heard that pulling the bed makes any work on either tank much easier.

 

You can pull the bed easily enough with the tank in place. It's really a little scary how little time it takes to pull the bed. It's 6 bolts and one or two wiring harness connectors at the rear of the truck, unbolt both of the fuel fill tubes, and lift it off. Two guys of average strength can man-handle the bed easily enough, just have some saw horses or something handy to set it onto once it is off.

The only things that can really go wrong is if you aren't able to get the bed bolts loose. I think I just buzzed mine off from underneath with an impact wrench and they all came right out. I know they did change the style of bolts at some point though. They're supposed to all hold themselves in place and not spin, but I was thinking that one style or the other had more of a tendency to spin, and make you curse and throw things...

 

well, the bed nuts in my 96 were all completely seized to the bed bolts - even after several hours soaking in PB blaster, all bolts just spun in the bed so that I had to grind them all off - which involved dropping both tanks.

no big deal as i'm parting that truck out, but if i'm in the same boat, here, I think i'll just drive by the odometer on the front tank till I get to swapping my better bed onto this truck and repainting...

or if I drop the showerhead into the tank, i'll just curse up a storm and then drop the tank to get it out - if I can't reach my arm down inside :-)

 

Trunk monkey.....

 

what is a trunk monkey?

or don't I want to know?

 

- Road rage one (starts at about 0:33) is the best one. Just sayin' a properly trained monkey could reach into the hole and pull the showerhead out for you.

 

HA! nice! I can think of 100 more uses for such a vehicle feature, but an extra set of hands while working alone might be the best function - or would that be two extra sets of hands?